1. Field of the Disclosure
The present disclosure generally relates to a method of detecting indicator information transmitted through an indicator channel in a wireless communication system, and more particularly, to a method and apparatus for determining a threshold value, and detecting indicator information based on the determined threshold value.
2. Description of the Related Art
In wireless communication, various types of indicator channels exist. For example, a transmitter (for example, a base station) transmits acquisition indicator through an Acquisition Indicator Channel (AICH) in response to a Random Access (RA) procedure of a receiver (for example, a terminal). The acquisition indicator, which is a response to a random access preamble, may include, for example, Acknowledge (ACK), Negative-Acknowledge (NACK), and No response (NoResponse).
When the receiver receives indicator information from the transmitter, the receiver compares a received value with a threshold value, so as to decide what the indicator information is. That is, the receiver executes detection. The threshold value may be determined based on various criteria.
For example, a Constant False Alarm Rate (CFAR) detector uses a CFAR threshold value for detection, so as to satisfy a predetermined false alarm rate.
As another example, a Constant Missed Detection Rate (CMDR) detector uses a CMDR threshold for detection, so as to satisfy a predetermined missed detection rate (MDR).
As yet another example, a detection method determines a threshold value by taking into consideration both a false alarm rate and a missed detection rate. In this instance, when a threshold value that satisfies both a predetermined false alarm rate and a predetermined missed detection rate does not exist, the method may execute an exceptional process (for example, selecting a threshold value that preferentially satisfies one of a false alarm rate and a missed detection rate).
FIGS. 1A through 3B illustrate methods of determining a threshold value for detecting indicator information using a Probability Density Function (PDF) graph.
FIGS. 1A and 1B illustrate a CFAR detector that satisfies a Constant False Alarm Rate (CFAR), and determination of a threshold value for the CFAR detector.
FIG. 1A illustrates a case in which a PDF graph 100 of a received signal based on a binary hypothesis H0 (with 0 as a tested value) and a PDF graph 110 of a received signal based on a binary hypothesis H1 (with g as a tested value) overlap. A false alarm rate is a probability of determining the indicator information as ACK when the indicator information is Discontinuous Transmission (DTX), and a missed detection rate is a probability of determining the indicator information as DTX when the indicator information is ACK. In this instance, an area indicating the false alarm rate may be expressed by an area 106 enclosed by the horizontal axis 190, the vertical axis indicated by a threshold value η 102, and the PDF graph 100 of H0 on the right plane of the vertical axis indicated by the threshold value η (: “eta”) 102. In addition, an area indicating the missed detection rate may be expressed by an area 104 enclosed by the horizontal axis 190, the vertical axis indicated by the threshold value η 102, and the PDF graph 110 of H1 on the left plane of the vertical axis indicated by the threshold value η 102.
FIG. 1B illustrates a case in which a PDF graph 120 of a received signal based on H0 and a PDF graph 130 of a received signal based on H1 do not overlap. In this instance, an area indicating the false alarm rate may be expressed by an area 124 enclosed by the horizontal axis 190, the vertical axis indicated by a threshold value 122, and the PDF graph 120 of H0 on the right plane of the vertical axis indicated by the threshold value η 122. However, the area enclosed by the horizontal axis 190, the vertical axis indicated by the threshold value η 122, and the PDF graph 130 of H1 on the left plane of the vertical axis indicated by the threshold value η 122 is actually 0. That is, when the PDF graph 120 of H0 and the PDF graph 130 of H1 do not overlap, the area indicating the missed detection rate may not exist. Thus, when the PDF graph 120, indicating received information when ACK information is transmitted, and the PDF graph 130, indicating received information when DTX information is transmitted, do not overlap and are well separated, although the CFAR detector appears to be capable of determining a CFAR threshold value η (to be higher) so as to further decrease a false alarm rate without causing an increase in a missed detection rate, the CFAR detector actually does not adaptively determine a threshold value.
FIGS. 2A and 2B illustrate a CMDR detector that satisfies a Constant Missed Detection Rate (CMDR), and determination of a threshold value η for the CMDR detector.
FIG. 2A illustrates a case in which a PDF graph 200 of a received signal based on H0 and a PDF graph 210 of a received signal based on H1 overlap. In this instance, an area indicating a false alarm rate may be expressed by an area 206 enclosed by the horizontal axis 290, the vertical axis indicated by a threshold value η 202, and the PDF graph 200 of H0 on the right plane of the vertical axis indicated by the threshold value η 202. In addition, an area indicating a missed detection rate may be expressed by an area 204 enclosed by the horizontal axis 290, the vertical axis indicated by the threshold value η 202, and the PDF graph 210 of H1 on the left plane of the vertical axis indicated by the threshold value η 202.
FIG. 2B illustrates a case in which a PDF graph 220 of a received signal based on H0 and a PDF graph 230 of a received signal based on H1 do not overlap. In addition, an area indicating a missed detection rate may be expressed by an area 224 enclosed by the horizontal axis 290, the vertical axis indicated by a threshold value η 222, and the PDF graph 230 of H1 on the left plane of the vertical axis indicated by the threshold value η 222. However, an area enclosed by the horizontal axis 290, the vertical axis indicated by the threshold value η 202, and the PDF graph 220 of H0 on the right plane of the vertical axis indicated by the threshold value η 202 is actually 0. That is, when the PDF graph 220 of H0 and the PDF graph 230 of H1 do not overlap, the area indicating a false alarm rate may not exist. Thus, when the PDF graph 220, indicating received information when ACK information is transmitted, and the PDF graph 230, indicating received information when DTX information is transmitted, do not overlap and are separated, although the CMDR detector is capable of determining a CMDR threshold value (to be lower) so as to further decrease a missed detection rate without causing an increase in the false alarm rate, the CMDR detector actually does not adaptively determine a threshold value.
However, the CMDR detector of FIGS. 2A and 2B may be used for a case in which a quality level of received information, when indicator information is actually transmitted as ACK (Signal to Noise Ratio for ACK (SNRACK)), is estimated through side information from a higher layer or through blind estimation.
FIGS. 3A and 3B illustrate a detector that satisfies both a CFAR and a CMDR, and determination of a threshold value η for the detector.
FIG. 3A illustrates a case in which both a CFAR and a CMDR are satisfied. For example, in FIG. 3A, it is assumed that a threshold value 302 is determined to be a value corresponding to a half of an average value μ (corresponding to a gain of an ACK signal) of a PDF graph 310 of a received signal when H1.
FIG. 3B illustrates a case in which both the area indicating the missed detection rate and the area indicating a false alarm rate do not exist. When determining the threshold value η (=μ/2) 322 of a detector, the detector reduces a simple arithmetic sum of a CFAR and a CMDR (exceptional case) when the CFAR and the CMDR are not simultaneously satisfied. As another example, a threshold value may be determined to satisfy a predetermined one of the CFAR criterion and the CMDR criterion.
A method of merely applying a threshold value that satisfies both a predetermined CFAR and a predetermined CMDR may fail to determine an optimal threshold value. When a threshold value that satisfies both the predetermined CFAR and the predetermined CMDR does not exist, a method of determining a threshold value that satisfies one of the criteria or determining a threshold value of a detector that reduces the sum of a CFAR and a CMDR, may also fail to determine an optimal threshold value. Therefore, there is a desire for an improved method for determining a threshold value of a detector.
The schemes for determining a threshold value of a detector, described through FIGS. 1A through 3B, determine a threshold value to be applied for determining indicator information by taking into consideration a false alarm rate and a missed detection rate, separately or together. The determining schemes simply assume that costs incurred by a false alarm event and costs incurred by a missed detection event are same, and simply take into account minimizing a false alarm rate and a missed detection rate or minimizing the sum of the false alarm rate and the missed detection rate. Thus, optimizing the performance of a detector has not been fulfilled. That is, the detector described in FIGS. 1A through 3B does not take into account actual costs incurred by the false alarm rate and the missed detection rate. For example, for a case of an AICH of Wideband Code Division Multiple Access (WCDMA), costs for a call setup delay may be significantly different between the false error event and the missed detection event.
Unlike traffic information of a data channel which is to reduce a symbol error probability, for indicator information of an indicator channel, determination of a threshold value needs to take into consideration costs since a large amount of cost may be incurred by a false alarm event or a missed detection event.